Definition of wh-word in English:

noun

Haj Ross named this process ‘pied piping’, conjuring an image of the wh-word luring the preposition out of its original position, just as the Pied Piper lured the rats and children out of Hamelin.

McGreevey's use was just that adjacency to a preposition seems to make the insertion of whom somewhat more likely, presumably because in many cases, the wh-word would really be the object of the preposition.

As far as I know, every instance of a double-is construction has a plausible (fully grammatical) alternate with an overt wh-word; but the mapping doesn't always work in the other direction.